The instant invention relates to detectors for microwave radiation and, more particularly, to radar antennas sensitive to the polarization of the received signal.
The utilization of microwave radiation to locate and track moving objects has been known for many years and forms the basis of the many active radar systems, ranging from those tracking spacecraft to hand-held police speed-measuring units for automobiles. Such units typically emit a train of pulses at microwave frequencies from an emitting antenna and use the same antenna--generally in the form of a paraboloid--to detect echoes reflected from the target. Such units use (a) the time delay, between a given pulse in the transmission and its echo, to determine distance, (b) the Doppler effect to determine speed, and (c) antenna position to determine the line of bearing (LOB) to the target.
The instant invention addresses itself to an additional item of information which may be derived from a reflected signal, or, a priori, from a signal purposely or intrinsically emitted by the target being tracked. This information relates to the polarization of the received microwave beam. Electromagnetic radiation can be viewed as composed of two orthogonally superimposed trains of waves, e.g., by referring to such trains as being horizontally and vertically polarized in a terrestrial frame of reference. It is possible, in the case of a deliberately emitted signal beam, to define the proportion of energy sent out in the horizontal component and in the vertical component independently, by appropriately shaping the emitting conductor or array.
In the case of a reflected signal, the shape and movement of the reflecting body--and in some instances its physical make-up--can alter the polarization pattern of the radiation returned to the illuminating antenna. Whatever the reason for the variation in the polarization of a microwave signal, the knowledge of this factor can help to identify and characterize the body which is emitting it, either actively or by reflecting a beam originating elsewhere.
Conventional radar units,.being co-polar in both transmit and receive modes, cannot distinguish between different polarization patterns of incoming radiation. The commonly utilized parabolic antenna destroys this information by focusing the signal onto a single receiver, or feed; and other antenna forms, even if more sensitive to particular directions of polarization cannot, in the absence of a directional reference, provide meaningful information on the source.
Since it is a prime purpose of most radar microwave receivers to derive angle and range information on a particular target, the prior art has failed to provide any effective devices or components of a practicable nature to perform the task of differentiating polarization patterns. Proposals to employ dual-polarized feeds in conjunction with an orthomode junction to separate orthogonally polarized signals into separate channels for further processing have not been a requirement of radar systems.
The requirement for polarization information is most acute in the fields of passive sensing--associated with electronic warfare, countermeasures, reconnaissance and surveillance systems--where information is sought on all characteristics of a detected signal, including frequency, band width, pulse width, repetition rate, as well as polarization data.